Antibodies to the enzyme myosin light chain kinase have been purified, characterized and used to study the role of this enzyme in regulating smooth muscle contraction as well as contractile activity in non-muscle cells. Immunofluorescence studies have demonstrated that myosin kinase is localized on stress fibers of cultured cells. Data from experiments in which living cells were microinjected with there antibodies suggest that myosin kinase plays an important role in regulating contractile activity in non-muscle cells.